dev-setup: allow a path prefix for use in chroots

With this adjustment, we can reuse this code elsewhere, such as in
nspawn.

src/core/mount-setup.c

@@ -398,7 +398,7 @@ int mount_setup(bool loaded_policy) {
         /* Create a few default symlinks, which are normally created
          * by udevd, but some scripts might need them before we start
          * udevd. */
-        dev_setup();
+        dev_setup("");
 
         /* Mark the root directory as shared in regards to mount
          * propagation. The kernel defaults to "private", but we think